1. Technical Field
The present invention relates to a filtration unit which is provided in a water storage tank, water storage pond such as a settling pond in a water cleaning plant, etc. in order to perform a filtration treatment on raw water; and the present invention further relates to an installation method for such a filtration unit and to a filtration apparatus in which this filtration unit is installed.
2. Description of the Related Art
As filtration apparatuses of the type described above, apparatuses in which a filtration module constructed by means of a hollow filament membrane or the like is immersed in a tank into which raw water is introduced, and aeration pipes that spread air inside the tank are installed beneath this filtration module, have been proposed in, for example, Japanese Patent Application Laid-Open (Kokai) Nos. H8-57273, H9-24250, H10-323683 and 2003-94056 and in Japanese Patent No. 3440402.
In the filtration apparatuses in the above prior arts, the filtration efficiency is improved by adding active carbon to the raw water that is introduced into the tank, or immersing or disposing active carbon inside the tank, and causing organic matter, etc., contained in the raw water to be adsorbed by this active carbon; moreover, the active carbon that has thus adsorbed organic matter, etc., contained in the raw water settles to the bottom of the tank and is extracted, or is discharged together with the raw water that is contained in the tank.
However, in the filtration apparatus disclosed in Japanese Patent Application Laid-Open (Kokai) No. H9-24250, active carbon is added to the raw water itself that is introduced into the tank (vessel). In the filtration apparatus shown in Japanese Patent No. 3440402, active carbon is immersed in the actual treatment tank into which the raw water itself is introduced.
Accordingly, in both apparatuses described above, a large amount of active carbon must be added relative to the amount of raw water that is introduced in order to achieve a sufficient improvement in the filtration efficiency by means of this active carbon. As a result, the cost of the apparatus increases. Furthermore, since mud, etc., is still contained in large quantities in the raw water that is introduced into such a tank, the active carbon that is extracted from the bottom of the vessel (especially as in Japanese Patent Application Laid-Open (Kokai) No. H9-24250) is in a state in which the active carbon is mixed with this mud and must be treated “as is” as a waste product.
Meanwhile, in the filtration apparatuses described in Japanese Patent Application Laid-Open (Kokai) Nos. H8-57273, H10-323683 and 2003-94056, raw water from which such mud has been separated beforehand to some extent in a precipitation and separation tank, etc., is introduced into a separate treatment tank, etc., and filtered by means of active carbon and a filtration module. In this case, however, two tanks are required, and thus a large space is required for installing the filtration apparatus.